U.S. patent number 4,536,658 [Application Number 06/455,715] was granted by the patent office on 1985-08-20 for hybrid schottky infrared focal plane array.
This patent grant is currently assigned to The United States of America as represented by the Secretary of the Air. Invention is credited to Charlotte E. Ludington.
United States Patent |
4,536,658 |
Ludington |
August 20, 1985 |
Hybrid Schottky infrared focal plane array
Abstract
A two dimensional focal plane array of Schottky internal
emission photodiodes which sense radiation in their metallic
regions on a silicon substrate for infrared imaging. The array is
designed for mating with multiplexing circuitry via hybrid bump
bonding techniques.
Inventors: |
Ludington; Charlotte E.
(Sherborn, MA) |
Assignee: |
The United States of America as
represented by the Secretary of the Air (Washington,
DC)
|
Family
ID: |
23809991 |
Appl.
No.: |
06/455,715 |
Filed: |
January 5, 1983 |
Current U.S.
Class: |
250/208.1;
250/332; 250/370.08; 250/370.14; 257/453; 257/E27.161 |
Current CPC
Class: |
H01L
27/14881 (20130101) |
Current International
Class: |
H01L
27/148 (20060101); H01J 040/14 () |
Field of
Search: |
;357/15,71,29,30,32
;250/211J,578,332,330,338 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Nelms; David C.
Attorney, Agent or Firm: Singer; Donald J. Donahue; Richard
J.
Government Interests
STATEMENT OF GOVERNMENT INTEREST
The invention described herein may be manufactured and used by or
for the Government for governmental purposes without the payment of
any royalty thereon.
Claims
What is claimed is:
1. A hybrid Schottky infrared internal photoemission focal plane
array comprising:
a first semiconductor substrate having a first surface and an
opposed second surface, said first semiconductor substrate being
formed of silicon material;
a plurality of infrared sensitive metallic photodiodes arranged in
a two-dimensional mosaic pattern on said first surface of said
first semiconductor substrate;
each of said plurality of infrared sensitive metallic photodiodes
comprising platinum silicide material formed at said first surface
of said first semiconductor substrate to establish a Schottky
potential barrier at the interface of said platinum silicide
material and said silicon material of said first semiconductor
substrate;
each of said plurality of infrared sensitive metallic photodiodes
having a metallic bonding pad formed on said platinum silicide
material for deriving an output signal therefrom;
each of said plurality of infrared sensitive metallic photodiodes
being surrounded by a guard ring having a width of two to eight
micrometers;
a channel stop having a width of one to five micrometers disposed
between each said guard ring; and
a second semiconductor substrate having a first surface and an
opposed second surface, said second semiconductor substrate being
formed of silicon material;
said second semiconductor substrate having multiplexing circuitry
including a two-dimensional matrix of metallic signal input
terminals formed on said first surface thereof for receiving and
multiplexing input signals;
said metallic bonding pad of each of said plurality of infrared
sensitive metallic photodiodes being directly and conductively
bonded to a different one of said matrix of metallic signal input
terminals of said second semiconductor substrate, whereby signals
derived from each of said plurality of infrared sensitive metallic
photodiodes are directly coupled to said multiplexing circuitry of
said second semiconductor substrate.
Description
BACKGROUND OF THE INVENTION
The present invention relates generally to a sensing surface, or
focal plane, for an infrared camera. More particularly, the present
invention relates to a hybrid focal plane array formed of densely
packed Schottky infrared internal emission photodiodes.
In U.S. Pat. No. 3,902,066, issued Aug. 26, 1975 to Roosild et al,
there is described a monolithic Schottky barrier array for
detecting the infrared portion of the electromagnetic spectrum.
Individual Schottky electrodes within the array are connected
through enhancement mode field effect transistors to charge coupled
devices for providing signals to an infrared vidicon camera. The
performance of this array has proven to be limited by aliasing and
drop-out effects which are related to the low percentage active
area of the array. Space is required within the focal plane for
multiplexing the signals derived from the individual Schottky
barrier electrodes, and also for the guard rings which are provided
about each individual Schottky electrode. In such an array, the
charge coupled device signal readout circuitry utilizes up to 50%
of the focal plane area. Add to this losses of sensing area because
of guard rings and channel stops and the active area of the array
is reduced to approximately 30 to 45 percent of the total area of
the array.
SUMMARY OF THE INVENTION
In accordance with the present invention, it has been found that
Schottky internal photoemission focal plane mosaics, which sense
infrared radiation via emission from a metallic electrode array and
have 70% active area coverage can be realized by hybrid bump
bonding to a separate integrated circuit multiplexer chip. Although
such hybridization techniques have been successfully demonstrated
in the bonding of multiplexer circuitry to arrays which sense
infrared radiation in the semiconductor, as taught in U.S. Pat. No.
3,808,435 issued to Robert T. Bate et al, they have not been
considered suitable for arrays which sense infrared radiation in
metal.
Accordingly, the primary object of this invention is to provide an
improved Schottky barrier focal plane array.
A further object of this invention is to provide a hybrid Schottky
barrier array having an increased active sensing area.
These and other advantages, features and objects of this invention
will become more apparent from the following description when taken
in connection with the illustrative embodiments in the accompanying
drawings.
DESCRIPTION OF THE DRAWINGS
FIG. 1 is a pictorial representation of the basic elements of an
infrared camera system.
FIG. 2 is a diagrammatic representation of prior art Schottky
infrared focal plane array.
FIG. 3 is a diagrammatic representation of a hybrid focal plane
array formed in accordance with the present invention.
FIG. 4 is a diagrammatic representation of a basic Schottky
photodiode without a guard ring.
FIG. 5 is a diagrammatic representation of prior art Schottky
photodiodes having guard rings.
FIG. 6 is a diagrammatic representation of a portion of an improved
Schottky hybrid focal plane array formed in accordance with the
present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring now to FIG. 1 of the drawings, there is shown a pictorial
representation of the basic elements of a camera for infrared
imaging wherein an infrared scene 2 is projected onto a
semiconductor focal plane 4 by means of optics 6. Focal plane 4
includes an array of infrared sensitive photodiodes 8 in a two
dimensional mosaic pattern. During an exposure or frame time a
charge image is built up on the mosaic that is a replica of the
observed scene. At the end of the frame, the charge image is read
out as a video signal by means well known in the art.
FIG. 2 represents the component layout configuration of a prior art
Schottky infrared focal plane array 10 such as that shown in U.S.
Pat. No. 3,902,066 mentioned above. This is a monolithic device in
which both sensing and multiplexing takes place within the focal
plane. The array includes a number of infrared detectors 12, each
having a guard ring 14 thereabout to minimize edge breakdown
leakage. Channel stops 16 are used to isolate individual
photodiodes from each other and from the charge coupled multiplexer
network 18 which is used for signal readout. In such prior art
devices, the charge coupled devices alone utilize up to 50% of the
focal plane area.
FIG. 3 represents a hybrid focal plane array assembly 20 formed in
accordance with the present invention in which a simple Schottky
focal plane array 22 is bump-bonded to a separate multiplexer chip
24. Methods of making suitable bump bonds 26 to the Schottky
photodiodes 28 are known in the art.
FIG. 4 represents the schematic of a basic Schottky photodiode 30
including a substrate semiconductor 32, a Schottky metal
photoelectrode 34 and an insulator 36. When the photodiode is
fabricated, the differences in chemical potential between the metal
and the semiconductor causes free charge to be swept from the
semiconductor region that is immediately adjacent to the electrode.
This process creates both the Schottky potential barrier at the
metal-semiconductor interface and a high field depletion region 38
in the semiconductor. Electric field strengths at the edge of the
Schottky electrode 40 can be extremely high and it is in this
region where most device limiting dark currents and breakdown
effects occur.
FIG. 5 illustrates the conventional means for eliminating edge
related dark currents. Guard rings 42 having widths in the order of
2 to 8 micrometers are formed by means of impurity diffusion or ion
implantation at the edges of the photoelectrodes. Further, to
prevent short circuiting between adjacent electrodes, a space or
channel stop 44 is provided between guard rings. The depletion
region is now confined to the area 46. As illustrated in FIG. 5,
for a detector active area having dimension d, a guard ring width
g, and a channel stop width c, the mosaic percent active area will
be:
It can therefore be seen that using channel stop widths of 1 to 5
micrometers, and guard ring widths of 2 to 8 micrometers, a percent
active area of better than 70% is realized for typical device
geometrics.
FIG. 6 illustrates a portion of the improved hybrid focal plane
array using guard ring structures. There the individual Schottky
electrodes 48 are surrounded by guard rings 50. A spacing or
channel stop 52 is provided between adjacent Schottky electrodes.
To complete the structure oxide feed throughs 54 connect to bumb
bonding pads 56 and substrate ohmic contacts are added by means
well known in the art. A second semiconductor substrate containing
multiplexing circuitry is then bonded to each of the bump landing
pads. The semiconductor base material of the present invention is
preferrably silicon while the Schottky electrodes are preferrably
formed of platinum silicide or similar metal silicides.
Although the invention has been described with reference to a
particular embodiment, it will be understood to those skilled in
the art that the invention is capable of a variety of alternative
embodiments within the spirit and scope of the appended claims.
* * * * *